We are taught that light behaves both like a particle and like a wave. We think of the photon as the particle form of light. Question: When a photon hits your eye as you look at the sky, or directly at the sun (through a really dark glass), is that the same photon that left the sun? Or does light operate more like electricity? The electron that powers my desk computer is just the result of a series of bumping electrons that eventually push a local electron through my power supply.

It depends on what that photon has been through – and in this case I mean that literally. Let’s start with the simple case, which in this case means I’m going to temporarily put you in space with invincible eyeballs to stare at the sun. In this case, the photon of light has left the sun, and travelled through the vacuum of space, and hit your eyeball. The photon that hits your eye is the same as the photon that left the surface of the sun. This is pretty straightforward, as there isn’t anything that could have interfered with the beam of light making it to your eyes.

Now, we can remove your eyeball invincibility, and give you back your dark glasses – welder’s glass is best for this sort of thing. Now the light has a bit of a barrier to make it through. The difference is not that the photons are pushing each other along like a current to make it through the dark glass, but that the vast majority of the photons don’t make it through the glass. That opacity is what makes it safe for you to view the sun this way; you’re not blasting your eyeballs with too much light. But the photons which do make it through should be the same as the ones that left the sun, something like eight minutes earlier. If I also put you back on the surface of the earth, there will be effectively no difference in how the photons behave. The atmosphere plays very little role in this story; by nature of us being able to see through it, it doesn’t affect visible light very much.

However, the constancy of photons from the sun doesn’t hold for all visible light photons that might make it to your eye. A photon of light can be absorbed by a material (say, dust, or a glow-in-the-dark dinosaur), and that material will temporarily gain the energy that had been held in the photon of light. That energy usually gets dumped back out again in the form of a new photon, and depending on the material that absorbed the light, that new photon can be in the visible. A glow-in-the-dark dinosaur, to stick with that example, absorbs ultraviolet light, and then radiates it back out in the visible over a period of time.

So if you’re looking up at the night sky or the daytime sky, those photons making it to your eye should be the exact same photons that left their respective stars. However, if you’re looking at something which is glowing under a blacklight, or a glow-in-the-dark toy, those photons are different from the ones that left the blacklight.